Abstract:

An ultraviolet light sensor includes an elongated metal oxide
nanostructure, a layer of an ultraviolet light-absorbing polymer, a
current source and a current detector. The elongated metal oxide
nanostructure has a first end and an opposite second end. The layer of an
ultraviolet light-absorbing polymer is disposed about at least a portion
of the metal oxide nanostructure. The current source is configured to
provide electrons to the first end of the metal oxide nanostructure. The
current detector is configured to detect an amount of current flowing
through the metal oxide nanostructure. The amount of current flowing
through the metal oxide nanostructure corresponds to an amount of
ultraviolet light impinging on the metal oxide nanostructure.

Claims:

1. An ultraviolet light sensor, comprising:a. an elongated metal oxide
nanostructure having a first end and an opposite second end;b. a layer of
an ultraviolet light-absorbing polymer disposed about at least a portion
of the metal oxide nanostructure;c. a current source configured to
provide electrons to the first end of the metal oxide nanostructure;
andd. a current detector configured to detect an amount of current
flowing through the metal oxide nanostructure,wherein the amount of
current flowing through the metal oxide nanostructure corresponds to an
amount of ultraviolet light impinging on the metal oxide nanostructure.

2. The ultraviolet light sensor of claim 1, further comprising:a. a metal
contact coupled to the first end of the metal oxide nanostructure, the
metal contact comprising a metal that creates a Schottky barrier between
the metal contact and the metal oxide nanostructure; andb. an Ohmic
contact coupled to the second end of the metal oxide nanostructure.

6. The ultraviolet light sensor of claim 2, wherein the ultraviolet
light-absorbing polymer comprises polystyrene sulfate, the ultraviolet
light sensor further comprising a layer of a polymer having a positive
surface charge disposed at least intermittently between the metal oxide
nanostructure and the polystyrene sulfate.

9. An ultraviolet light sensing element, comprising:a. an elongated metal
oxide nanostructure having a first end and an opposite second end; andb.
an ultraviolet light-absorbing polymer disposed so as to envelope the
metal oxide nanostructure.

10. The ultraviolet light sensing element of claim 9, further
comprising:a. a metal contact coupled to the first end of the metal oxide
nanostructure, the metal contact comprising a metal that creates a
Schottky barrier between the metal contact and the metal oxide
nanostructure; andb. an Ohmic contact coupled to the second end of the
metal oxide nanostructure.

14. The ultraviolet light sensing element of claim 13, wherein the
ultraviolet light-absorbing polymer comprises polystyrene sulfate, the
ultraviolet light sensor further comprising a layer of a polymer having a
positive surface charge disposed at least intermittently between the
metal oxide nanostructure and the polystyrene sulfate.

17. A method of making an ultraviolet light sensing element, comprising
the actions of:a. growing an elongated metal oxide nanostructure having a
first end and an opposite second end; andb. functionalizing a portion of
the metal oxide nanostructure with an ultraviolet light-absorbing
polymer.

18. The method of claim 17, further comprising the actins of:a. coupling a
metal contact to the first end of the metal oxide nanostructure, the
metal contact comprising a metal that creates a Schottky barrier between
the metal contact and the metal oxide nanostructure; andb. coupling an
Ohmic contact to the second end of the metal oxide nanostructure.

19. The method of claim 17, wherein the elongated metal oxide
nanostructure comprises a selected one of a zinc oxide nanobelt or a zinc
oxide nanowire.

20. The method of claim 19, wherein the functionalizing action
comprises:a. applying a polymer having a positive surface charge to a
surface of the selected one of a zinc oxide nanobelt or a zinc oxide
nanowire; andb. applying the an ultraviolet light-absorbing polymer to
both the selected one of a zinc oxide nanobelt or a zinc oxide nanowire
and the polymer having a positive surface charge.

21. The method of claim 20, wherein the functionalizing action further
comprises cleaning a surface of the zinc oxide nanobelt with an oxygen
plasma prior to the action of applying a polymer having a positive
surface charge to the surface of the zinc oxide nanobelt.

[0003]The present invention relates to electronic sensors and, more
specifically, to a sensor for ultra-violet light.

DESCRIPTION OF THE PRIOR ART

[0004]Ultraviolet (UV) photon detectors have a wide rang of applications
from environmental monitoring, missile launching detection, space
research, high temperature flame detection to optical communications. For
these applications, fast response time, fast reset time, high
selectivity, high responsivity, and good signal-to-noise ratio are
commonly desired characteristics. Silicon photodiodes, the most common
devices used as UV photodetectors, are strongly influenced by visible
light and they usually require filters to attenuate unwanted visible and
infrared (IR) radiations. For applications that require high-sensitively,
silicon photodiodes need to be cooled to a low temperature to reduce the
dark current.

[0005]For UV photon detectors based on polycrystalline zinc oxide thin
film, a slow response time ranging from a few minutes to several hours is
commonly observed. Due to large surface-to-volume ratio and reduced
dimensionality of the active area, zinc oxide nanostructures are expected
to have high photon conductance. Most of the studies have been focused on
the mechanism investigation and improving the sensitivity. However,
little attention has been paid on improving the response and recovery
time especially the reset-time (defined as the time need to recovery to
1/e (37%) of the maximum photocurrent), which may limit the applications
of zinc oxide nanowire nanosensors for fast UV detection and imaging.

[0006]Therefore, there is a need for an ultraviolet sensor that exhibits a
fast response time, a fast reset time, a high selectivity, a high
responsivity, and a good signal-to-noise ratio.

[0007]There is also a need for UV photodetectors that can be fabricated at
low cost and that are able to work at room temperature.

SUMMARY OF THE INVENTION

[0008]The disadvantages of the prior art are overcome by the present
invention which, in one aspect, is an ultraviolet light sensor that
includes an elongated metal oxide nanostructure, a layer of an
ultraviolet light-absorbing polymer, a current source and a current
detector. The elongated metal oxide nanostructure has a first end and an
opposite second end. The layer of an ultraviolet light-absorbing polymer
is disposed about at least a portion of the metal oxide nanostructure.
The current source is configured to provide electrons to the first end of
the metal oxide nanostructure. The current detector is configured to
detect an amount of current flowing through the metal oxide
nanostructure. The amount of current flowing through the metal oxide
nanostructure corresponds to an amount of ultraviolet light impinging on
the metal oxide nanostructure.

[0009]In another aspect, the invention is an ultraviolet light sensing
element. An elongated metal oxide nanostructure has a first end and an
opposite second end. An ultraviolet light-absorbing polymer is disposed
so as to envelope the metal oxide nanostructure.

[0010]In yet another aspect, the invention is a method of making an
ultraviolet light sensing element, in which an elongated metal oxide
nanostructure having a first end and an opposite second end is grown. A
portion of the metal oxide nanostructure is functionalized with an
ultraviolet light-absorbing polymer.

[0011]These and other aspects of the invention will become apparent from
the following description of the preferred embodiments taken in
conjunction with the following drawings. As would be obvious to one
skilled in the art, many variations and modifications of the invention
may be effected without departing from the spirit and scope of the novel
concepts of the disclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

[0012]FIG. 1 is a schematic diagram of one embodiment of an ultraviolet
light sensing element.

[0013]FIG. 2 is a block diagram showing a possible electron transport
mechanism occurring in the embodiment shown in FIG. 1.

[0014]FIG. 3 is a graph showing current as a function of time in an
experimental embodiment of a light sensing element as an ultraviolet
light source is cycled between "on" and "off"

[0015]FIG. 4 is a perspective view of an experimental embodiment of an
ultraviolet light sensor.

[0016]FIG. 5 is a perspective view of an embodiment of an ultraviolet
light sensing element that employs vertically-disposed nanowires.

[0017]FIG. 6A is an embodiment of an ultraviolet light sensing element
that employs a Schottky contact.

[0018]FIG. 6B is a schematic diagram of an ultraviolet light sensor that
employs the ultraviolet light sensing element shown in FIG. 6A

[0019]FIG. 7 is a flow chart demonstrating one method of making
ultraviolet light sensing element.

DETAILED DESCRIPTION OF THE INVENTION

[0020]A preferred embodiment of the invention is now described in detail.
Referring to the drawings, like numbers indicate like parts throughout
the views. Unless otherwise specifically indicated in the disclosure that
follows, the drawings are not necessarily drawn to scale. As used in the
description herein and throughout the claims, the following terms take
the meanings explicitly associated herein, unless the context clearly
dictates otherwise: the meaning of "a," "an," and "the" includes plural
reference, the meaning of "in" includes "in" and "on." Also as used
herein, "nanobelt" includes elongated nanostructures such as nanowires
and nanotubes.

[0021]U.S. Pat. Nos. 6,586,095, 6,918,959 and 7,220,310 and 7,351,607, all
issued to Wang et al., disclose methods for making metal oxide
nanostructures, the entirety of each of these patents is incorporated
herein by reference for the purpose of disclosing methods of generating
metal oxide nanostructures.

[0022]As shown in FIG. 1, one embodiment of an ultraviolet (UV) sensor 100
includes an ultraviolet light sensing element 110 coupled to a source 120
of current and an element 122 configured to sense current flowing through
the ultraviolet light sensing element 110. Typically, element 122 would
of a type selected from the many different types of solid-state current
sensors, depending upon the specific application.

[0023]The sensing element 110 includes an elongated metal oxide
nanostructure 112 (such as a nanobelt or nanowire) about which is
disposed a layer of a polymer 116 that absorbs UV light. In one
embodiment, the UV light-absorbing polymer includes polystyrene sulfate
(PSS). However, other UV light-absorbing polymers that may be employed
include poly(styrene-co-maleic acid) (PS-co-MAc), anionically charged
poly(N-isopropylacrylamide) (PNIPAM), carboxymethylcellulose (CMC).

[0024]A first contact 118 is applied to a first end of the nanostructure
112 and a second contact 119 is applied to a second end of the
nanostructure 112. In one embodiment, both contacts comprise an Ohmic
contact. In another embodiment, the first contact 118 comprises a metal,
such as platinum, so as to create a Schottky barrier between the first
contact 118 and the first end of the nanostructure 112, whereas the
second contact 119 includes a material that forms an Ohmic contact with
the second end of the nanostructure 112. In one embodiment, the second
contact 119 includes a Pt:Ga electrode.

[0025]PSS has a negative surface charge, as does zinc oxide. Therefore, in
the embodiment where the layer of a polymer 116 that absorbs UV light
includes polystyrene sulfate (PSS), an intermediate layer 114 of a
polymer having a positive surface charge is disposed at least
intermittently between the metal oxide nanostructure 112 and the layer of
UV light absorbing polymer 116. One example of a suitable polymer having
a positive surface charge includes poly(diallyl dimethyl-ammonium
chloride) (PDAMAC).

[0026]As shown in FIG. 2, it is believed that the absorption peak and the
related molecular energy states in PSS play a significant role for
enhancing the photon response of the sensing element 100. Although the
zinc oxide nanostructure 112 surface is covered by PDADMAC, it is well
known that the monolayer adsorption usually cannot reach 100% coverage.
Therefore, the remaining surface could be covered by the UV light
absorbing layer 116 of PSS. When subjected to UV light, an electron in
PSS is excited from the ground state energy state to an exited energy
state due to high absorption, which creates an unoccupied energy state.
If the ground energy state of the PSS is at the level within the band gap
of zinc oxide, it is possible that the electron in the valence band 204
of zinc oxide nanostructure 112 is likely to be excited to the ground
state of PSS, which subsequently transits to the conduction band 202 of
the zinc oxide nanostructure 112. This "hopping" process may largely
enhance the transition probability of the valence electrons in zinc oxide
to the conduction band 202, resulting in a large increase in the number
of electron-hole pairs. The role played by PSS is to serve as a
hopping-state or bridge for the electron transfer. The generated holes
may be trapped at the nanostructure surface by the PDADMAC, while the
electrons are transported through the nanostructure core. Therefore, the
conjunction of electron-hole pair generation in zinc oxide with the
assistance of PSS and surface hole trapping effect by PDADMAC may largely
prevent the electron-hole recombination, resulting in a substantial
increase in the photo-conductance of the PSS coated zinc oxide
nanostructure 112.

[0027]As shown in FIG. 3, in a current vs. time graph 300, one
experimental embodiment exhibits almost no current through the elongated
nanostructure during periods 310 when the sensing element was not
subjected UV light. However, a substantial increase in current flowed
through the nanostructure during periods 312 when the sensing element was
subjected to UV light.

[0028]One experimental embodiment, as shown in FIG. 4, employed a silicon
substrate 410 upon which was deposited a layer of silicon dioxide 412. A
polymer-functionalized nanobelt 110 was disposed on the silicon dioxide
layer 412 and two oppositely-disposed gold/titanium contacts were
deposited on the silicon dioxide layer 412 so as to be in contact with
the nanobelt 110.

[0029]An embodiment employing a plurality of vertically-disposed nanowires
510 are grown from a first end contact layer 516 and a second end contact
layer 518 is disposed adjacent the tops of the vertically-disposed
nanowires 510 so as to be in contact therewith. A substrate 512 may
provide a platform for the first end contact layer 516.

[0030]As shown in FIG. 6A, depositing a metal contact 618 (e.g., platinum)
at a first end of the nanostructure 110 so as to create a Schottky
barrier between the nanostructure 110 and the metal contact 618 reduces
reset time. An Ohmic contact 616 is deposited at the second end of the
nanostructure 110. This embodiment is shown schematically in FIG. 6B, in
which an electron source 620 has been added.

[0031]One method of making a UV light sensing element is shown in FIG. 7,
in which an elongated zinc oxide nanostructure is grown 710, typically on
a substrate. The nanostructure is cleaned 712 (e.g. with an oxygen
plasma) to remove impurities. A polymer having a positive surface charge
(e.g., PDADMAC) is applied to the nanostructure. A UV light absorbing
polymer (e.g., PSS) is then applied to the nanostructure 716 and a pair
of contacts is deposited at the ends of the nanostructure 718.

[0032]In one representative experimental embodiment, an effective way of
improving both the sensitivity and reset time of zinc oxide nanowire
nanosensors employed Schottky type (Schottky type) devices instead of
Ohmic type (OT) devices. In this embodiment, the UV sensitivity of zinc
oxide nanowire nanosensor was improved by four orders of magnitude, and
the reset-time was decreased from about 417 seconds to about 0.8 seconds.
By further surface coating with positive charged
poly(diallydimethylammonium chloride) (PDADMAC) and negative charged
poly(sodium 4-styrenesulfonate) (PSS), the reset-time was decreased to
about 20 milliseconds even without correcting the electronic response of
the measurement system.

[0033]The zinc oxide nanowires for the nanosensor fabrication were
synthesized by thermal evaporation of zinc oxide powders without using
any catalyst. UV response of the devices was characterized by a portable
UV lamp (Spectroline, Model ENF-280C, 365 nm). The photon-response
spectrum measurement was carried out in a PTI QuantaMaster Luminescence
(QM 3PH) system. All of the measurements were carried out at room
temperature in ambient condition.

[0034]First studied was the performance of an Ohmic-contact zinc oxide
nanowire nanosensor. To make an Ohmic contact, Ti/Au electrodes were
deposited on a single zinc oxide nanowire through shadow mask technology.
The high linear I-V characteristic of the device in darkness indicated
that the device was Ohmic as expected. By illuminating the device using a
365 nm UV source at a power density of about 30 μW/cm2, the
photon-conductance was improved by about 15%. After about 260 seconds
continuous illumination, the current was still unsaturated. More
importantly, the reset time of the sensor was about 417 seconds, and the
current could not recovery to its initial state even after about 2500
seconds.

[0035]The slow UV response and recovery of the Ohmic zinc oxide nanowire
nanosensor may be attributed to the oxygen adsorption and desorption
process. In darkness, oxygen molecules adsorb on the zinc oxide nanowire
surface by capturing free electrons from the n-type zinc oxide
[O2(g)+e-→O2-(ad)], thereby creating a
depletion layer with low conductivity near the surface. Upon UV
illumination at photon energies above the zinc oxide band gap,
electron-hole pairs are generated [hv.sup.→e-+h.sup.+].
Photon-generated holes migrate to the surface discharge the adsorbed
oxygen ions [O2-(ad)+h.sup.+→O2(g)] to
photon-desorbed oxygen from the surfaces. The unpaired electrons
accumulate gradually with time until desorption and re-adsorption of
O2 reach an equilibrium state, resulting in a gradual current rise
until saturation during UV illumination. Although holes recombine quickly
with electrons upon turning off UV light, there are still a lot of
electrons left in the zinc oxide. O2 molecules gradually re-adsorb
on the surface and capture these electrons, which results in a slow
current decay.

[0036]The sensitivity of Ohmic-type zinc oxide nanowire UV nanosensor can
be improved by using small size nanowires. However, the reset time is
still on the order of few hundred second or even longer. Such a
performance may not be adequate for sensor application especially used
for UV detection purpose at a high frequency.

[0037]The Schottky type zinc oxide nanowire nanosensors were fabricated by
the following process. First, patterned platinum microelectrode arrays
were fabricated on a SiO2/Si substrate by UV lithography and e-beam
evaporation process. Second, a single zinc oxide nanowire was placed on
the electrodes using a dry printing process. Finally, focus ion beam
microscopy was used to deposit a Pt:Ga electrode on one end of zinc oxide
nanowire to make a good Ohmic contact. Platinum was deposited on the
other end to form a Schottky contact.

[0038]The I-V characteristics of the Schottky type zinc oxide nanowire
nanosensor both in the dark and upon about 30 μW/cm2365 nm UV
light illumination indicated that the nanosensor was more sensitive when
the Schottky barrier was reversely biased. The response behavior of the
device was characterized by measuring the current under fixed bias of 1 V
(the Schottky barrier was reversely biased) as a function of time when
the device was periodically exposed to the UV light. The current
increased from 0.04 nA to 60 nA within 0.6 seconds, which is nearly
1500-fold enhancement in response. When the UV light was turned off, the
current decreased to reach its initial state within 6 seconds with a
reset-time of 0.8 seconds. The decay time of the photon-response follows
a second-order exponential decay function, with estimated time constant
of τd1=0.52 s and τd2=1.47 seconds, and relative weight
factors of 64% and 36%, respectively. The differences in device
performances between the two types of nanosensors can therefore attribute
to the Schottky barrier at the zinc oxide/Pt interface.

[0039]It's well known that metal Pt (work function of about 6.1 eV) and
n-type zinc oxide (work function of about 5.1 eV) can form a Schottky
contact, and the adsorbed oxygen at the metal/semiconductor interface can
significant modify the Schottky barrier. Normally, the presence of a
Schottky barrier at the metal/semiconductor interface plays a crucial
role in determining the electrical transport property of the structure.
At a fixed bias voltage, the voltage drop occurs mainly at the reversely
biased Schottky barrier. When the reverse-biased Schottky contact is
illuminated by 365 nm UV, photon-generated electrons and holes in the
interface region are separated by the strong local electric field and,
thus, reduce the electron-hole recombination rates and increase the
carrier lifetime, resulting in an increase in free carrier density. The
photon-desorption of oxygen at the zinc oxide/platinum interface modifies
the density of defects states and, hence, alters the Schottky barrier.
Both of the two processes may lower the height and narrow the width of
the Schottky barrier, thus electrons may transit over the lowered height
of the Schottky barrier or tunnel through the narrowed width of the
Schottky barrier. This mechanism accounts for the enhanced UV response.
The more rapid photocurrent decay in the Schottky type device is mostly
dictated by the electrical transport property of the Schottky barrier.
Upon turning off the UV light, the photon-generated electrons and holes
in the interface region decreased dramatically, and the oxygen is only
required to be re-adsorbed close to the interface to modify the Schottky
barrier height. It should be noted that, as the Schottky barrier can be
modified by many effects (such as surface absorption, strain, etc.), the
long time performance stability and reliability of the Schottky type
device may be affected, but surface passivation may protect the device.

[0040]The response speed of the Schottky type zinc oxide nanowire
nanosensor can be further enhanced by surface functionalization. Positive
charged PDADMAC and negative charged PSS layers were sequent coated on
the device by a layer-by-layer self-assembly method. The polymer coated
device has no response to visible light. However, a sharp response was
detected when the device was exposed to 365 nm UV light, indicating that
the Schottky type zinc oxide nanowire nanosensor is still UV selective
after surface coating. The current increased from 0.3 nA to 200 nA within
120 ms under about 30 μW/cm2UV illumination. When the UV light
was turned off, the current decreased to 37% of its initial photocurrent
within 110 ms. The decay time of the photon-response follows a
second-order exponential decay function, with estimated time constant of
τd1=0.084 s and τd2=0.88 s, and relative weight factors
of 88% and 12%, respectively. It should be noted that the response time
of the measurement system itself was set at 100 ms, thus the real reset
time the device should be much shorter than 110 ms.

[0041]The reset-time of the device was measured by setting the response
time as 10 ms. Although the noise level was high, the photon current
decreased from about 270 nA to about 50 nA in 20 ms, indicating that the
real reset-time of the device is less than 20 ms in the order of a few
ms.

[0042]The mechanism for giant improvement of the UV response speed by
surface coating is not fully understood yet. The polymer molecules may
largely occupy the sites at which the adsorbed and ionized oxygen tend to
occupy. Thus the UV response of device was dominated by the
photon-generated electrons and holes, the recombination of which in the
zinc oxide/platinum interface region is rather fast. But the oxygen
adsorption and desorption process are usually slow. It is known that
surface functionalization using polymers that have large UV absorption
peak can largely increase the UV response of the nanosensor. A rise in
response profile naturally improves the recovery time in a relatively
scale.

[0043]Utilizing of Schottky contacts and surface functionalization has
been demonstrated as an effective way for improving response speed
especially the reset-time of zinc oxide nanowire UV nanosensors. The fast
UV response speed, high spectrum selectivity combined with high
photosensitivity suggest the possibility of using zinc oxide nanowires as
"visible-blind" UV sensors for commercial, military, and space
applications. Beside the UV sensors, embodiments of the present invention
may also improve the performance of gas sensors, strain sensors and
biosensors by employing Schottky contacts introduced in device
fabrication, which is distinctly different from the conventionally
designed devices with Ohmic contacts.

[0044]The above described embodiments, while including the preferred
embodiment and the best mode of the invention known to the inventor at
the time of filing, are given as illustrative examples only. It will be
readily appreciated that many deviations may be made from the specific
embodiments disclosed in this specification without departing from the
spirit and scope of the invention. Accordingly, the scope of the
invention is to be determined by the claims below rather than being
limited to the specifically described embodiments above.